This has already been reported but it was closed as invalid due to "being a feature request" but this isn't invalid, it's an actual issue. My best guess would be that they forgot to add them. It is completely inconsistent with other smooth blocks, and every other block that has a slab variant, and it is something that needs to be fixed.

The bug

What was expected

That smooth stone would have a stair variant, just like every other block that has a slab variant.

How it is

Smooth stone, unlike any other block that has a slab variant, does not have a stair variant.